A fluidized bed is a suspension of solid particles in a stream of gas or liquid of sufficient velocity to support the particle by flow forces against the downward force of gravity. Fluidized beds are critical components of important petrochemical processing units such as the catalytic cracking ("cat-cracking") of petroleum on catalytic particles to produce lighter and more valuable products as well as thermal cracking of heavy feeds on coke particles ("fluid bed cokers" or "flexi-cokers") to again produce lighter and more valuable feeds. In cat-cracking the regenerator where coke is burned off the catalyst to produce "fresh catalyst" contains a fluidized bed. The particles in the fluidized bed within the regenerator are approximately 60 micron diameter pellets of a zeolite. In the case of fluid bed or flexi-coking, fluidized beds can be found in the heater, reactor and in the case of flexi-coking, the gasifier. The particles in this case are approximately 100 to 150 micron particles of coke.
Other fluidized beds containing small solids suspended in a gas include advanced coal combustion units where small particles of coal are suspended and burned to produce heat with minimum pollution and maximum efficiency. Yet another example is found in separation processes in the chemical industry where a fine suspension of particles is suspended in a flowing liquid. In general, fluidized beds are used in many large scale processes where it is desired to maximize the interaction between the surface of a particle and a surrounding gas or liquid.
Fluidized beds can contain volume mass densities for the case of fluid bed cokers and regenerators of the order of 40 pounds per cubic foot and particle velocities of several feet per second. Fluidized beds of the order of 10 to 50 feet in diameter are found in coking and cat cracking. With bed heights of the order of 10 to 60 feet, the contained fluids range from less than a hundred to more than a thousand tons of particles.
It is desirable to be able to directly measure the characteristics of fluidized bed reactors. Such characteristics include bed level location, transfer line operation, feed-injector operation, flow uniformity through the bed, uniformity as a function of time, and, for cokers, wall-coke thickness as coke builds up on the interior wall of the unit.